More particularly, the invention relates to a hinge mechanism comprising:                first and second metal frames mounted rotatably relative to each other, the first frame comprising a cylindrical neck of circular section,        a control member mounted rotatably in the neck of the first frame, said control member being capable of controlling the relative rotation of the first and second frames,        a metal retaining part, capable of retaining the control member at least in a first direction parallel to the first axis.        
Document DE-A-10 2008 028 102 describes an example of a hinge mechanism of this type, in which the control member passes through the second frame and has an external part that protrudes out of the second frame opposite the first frame, this external part comprising a radial groove in which the retaining ring is elastically fitted.
This hinge mechanism has the following drawbacks:
                the control member is made longer, heavier and more complex due to the mounting of the retaining ring,        the hinge mechanism is made more bulky in the axial direction due to the protruding external part of the control member,        the metal retaining ring rests against the outer surface of the second metal frame, which can generate friction and noise during operation of the hinge mechanism, in such a way that in practice it is necessary to provide a sliding block between the retaining ring and said outer surface of the second frame, leading to additional cost.        
A particular purpose of this invention is to overcome these drawbacks.